This invention relates to an improved process for converting non-gaseous carbonaceous materials to gaseous carbonaceous materials. More particularly, this invention relates to an improved process for gasifying normally solid carbonaceous materials.
Heretofore, several processes have been proposed for converting solid carbonaceous materials to gaseous products which may then be used either as fuels or as feeds to various processes such as the Fischer-Tropsch process. In these processes, the solid carbonaceous material is contacted with a suitable gasification agent at elevated temperatures. The processes may be either thermal or catalytic. Suitable gasification agents include hydrogen, steam, carbon dioxide, synthesis gas and the like.
Of these several processes, the catalytic processes, particularly those wherein an alkali metal catalyst is employed, would appear to offer the greater advantages since the catalytic processes may be operated at significantly lower temperatures thereby reducing the amount of heat required to effect the conversion. Moreover, lower temperature operations tend to favor the production of methane which is frequently a particularly preferred gasification product.
As is well known, an extensive research effort has been completed on catalytic gasification processes wherein an alkali metal hydroxide or an alkali metal salt is used as a catalyst or catalyst precursor. As is also well known, processes of this type would appear to offer tremendous advantages particularly in the gasification of solid carbonaceous materials. More recently, however, it has been learned that the activity of the alkali metal catalysts decrease as the conversion proceeds when steam is used as the gasification agent at elevated pressures; i.e., pressures above about 100 psig. This reduced catalytic activity results in a reduced conversion of carbon to the desired gaseous products at any given holding time thereby detracting from the other process advantages. Heretofore, it had been believed necessary to either use larger gasifiers, higher temperatures, higher steam rates or a combination of these to offset the lower conversion associated with the catalytic activity. All of these, however, lead to increased costs. The need, then, for an improved process wherein the reduction in catalytic activity is either eliminated or reduced is believed readily apparent.